1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to an improvement of a bead portion structure in a heavy duty pneumatic radial tire capable of effectively preventing the pulling-out of carcass ply cord, occurrence of separation failure at turnup portion of carcass ply and the like to improve the durability. Further, it relates to a method of manufacturing such a pneumatic tire as well as a carcass band bending apparatus used therefor and a tire manufacturing apparatus.
2. Description of Related Art
In the conventional heavy duty pneumatic radial tire, the carcass ply is toroidally extended between a pair of bead cores in bead portions and turned up around the bead core from an inside of the tire toward an outside thereof in a radial direction and the resulting turnup portion is embedded and fixed in rubber in order to prevent the pulling-out of the carcass ply cord during the running of the tire under loading.
FIGS. 1a and 1b show diagrammatically section views of typical embodiments of the bead portion in the conventional heavy duty pneumatic radial tire, respectively. In the illustrated embodiment of FIG. 1a, an outer end of a turnup portion 1a of a carcass ply 1 wound around a bead core 3 is located outward from an outer end of a wire chafer 2 in a radial direction of the tire. In the illustrated embodiment of FIG. 1b, the outer end of the wire chafer 2 is located outward from the outer end of the turnup portion 1a of the carcass ply 1 in the radial direction. Moreover, numeral 4 is a bead filler.
In such conventional bead portion structures, however, the difference of stiffness is produced between inside and outside in the radial direction of the tire on the border of the position of the outer turnup end of the carcass ply 1 or the outer end of the wire chafer 2, a zone ranging from the bead portion to the sidewall portion is subjected to repetitive deformation during the running of the tire under loading, whereby stress is concentrated at each of the outer ends and in the vicinity thereof and hence it is apt to cause separation failure at the outer end from rubber, which results in the occurrence of crack cr in the bead portion as shown in FIGS. 2a and 2b. 
In order to mitigate stress produced at the outer turnup end of the carcass ply 1 or the outer end of the wire chafer 2 and in the vicinity thereof and enhance the stiffness of the bead portion to control the deformation of the bead portion, therefore, there are proposed a method wherein plural organic fiber cord layers (not shown) are circumscribed with the wire chafer 2 around the bead core 3 so as to cover the outer turnup end of the carcass ply 1 or the outer end of the wire chafer 2, a method of increasing an amount of the bead filler 4, particularly hard rubber filler arranged at the outside of the bead core 3 between a main body of the carcass ply 1 and the turnup portion thereof in the radial direction, and the like. According to these methods, however, the heat generating temperature of the bead portion becomes more higher during the running of the tire under loading, which results in the occurrence of separation failure even at the outer end of the organic fiber cord layer in addition to the above separation failure, and also the tire weight is undesirably increased to lower the productivity of the tire.
When the heavy duty pneumatic radial tire is used several times by recapping after the wearing, there is a further problem that it is impossible to use the tire due to the occurrence of separation failure around the turnup end of the carcass ply subjected to stress concentration in the use over a long time.
As a countermeasure for solving such a problem, there is a technique that the rubber gauge of the bead portion is reduced by forming a recess portion in an outer profile of the bead portion at the radial section of the tire to lower the heat build-up of the bead portion (for example, JP-A-57-191104). In this technique, however, the rubber gauge of the bead portion can not be reduced too much owing to the presence of the turnup portion of the carcass ply, so that the effect of improving the bead portion durability can not be said to be sufficient.
Furthermore, JP-A-10-193924 proposes a technique that the thickness of the bead portion is decreased by extending the turnup portion along the main body of the carcass ply to form a recess portion in the outer profile of the bead portion. In this case, however, shearing strain is caused in an interface between the main body and the turnup portion of the carcass ply is increased by the shearing deformation based on the pushing from the rim flange under loading as the rubber gauge at the outside of the turnup portion is decreased and hence there is caused a problem that separation failure is caused between the carcass ply and rubber located at the outside thereof.
In any case, it is insufficient to prevent the separation failure in the bead portion even by these conventional techniques because the service conditions of the large-size pneumatic radial tire become recently severer with the increase of the recapping number from viewpoints of environmental consideration and economical merit and the bead portion durability is degraded in lower section-profile tires recently used, and hence basic solution for bead portion troubles are not yet obtained.
In the manufacture of the tire as mentioned above, when the winding of the turnup portion of the carcass ply around the peripheral surface of the bead core is carried out by subjecting the ply cords and hence the carcass ply to elastic deformation in the winding direction with, for example, well-known bladder, blade, roll and the like and then sticking the elastic deformed turnup portion onto the peripheral surface of the bead core with rubber, it is difficult to wind the ply cords onto the peripheral surface of the bead core with a high accuracy as is expected. And also, ply cords having a large elastic restoring force such as steel cords, aromatic polyamide cords and so on can not accurately be maintained at the required winding position irrespectively of the ply cord having a small elastic restoring force. In any case, there is a problem that the pulling-out of the ply cord and the separation failure at the turnup end of the carcass ply can not sufficiently be prevented, respectively.